Low Temperature Poly-Si on Flexible Polymer Substrates for Active Matrix Displays and Other Applications
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Low Temperature Poly-Si on Flexible Polymer Substrates for Active Matrix Displays and Other Applications Nigel D. Young, Michael J. Trainor, Soo-Young Yoon, David J. McCulloch, Richard W. Wilks, Andrew Pearson, Sandra Godfrey, Peter W. Green, Sander Roosendaal1 and Elizabeth Hallworth Philips Research Laboratories, Redhill, RH1 5HA, U.K. 1 Philips Research Laboratories, Eindhoven, 5656AA, The Netherlands. ABSTRACT A variety of polymer materials including polyimide (PI), polyarylate (PAR), polynorbonene (PNB) and polyethersulphone (PES) have been studied for use as substrates in the formation of active matrix displays based upon polycrystalline silicon (poly-Si) thin film transistors (TFTs). A process used to fabricate transflective mobile phone displays at 250oC on such substrates is described in detail. The NMOS TFTs show a mobility of 100cm2/Vs, and a threshold voltage of 3.9V; the PMOS devices have a mobility of 52cm2/Vs, and a threshold voltage of -6V. Issues relating to performance of these devices, yield of the arrays, and manufacturability are discussed.
INTRODUCTION
There is increasing interest in using polymer or metal foil substrates rather than glass substrates for a wide range of display applications, for two main reasons: (i) in order to produce thinner, lightweight and robust replacements for glass displays, and (ii) in order to produce highly flexible or even rollable displays. We will discuss here the various applications for ‘glass replacement’ and ‘flexible’ displays separately, and will consider the various technologies which have to be examined for each of these applications. Glass replacement displays are most attractive for small, mobile applications where low weight, low power and small footprint are important requirements. Even lap top displays could make advantage of such lightweight displays, but the impact here is rather less significant. The key mass-market applications are for mobile phones, cameras, and PDAs, though several new applications are emerging, such as electronic paper and electronic books. It has become apparent over the last year or two that all of these applications are moving towards active matrix addressing for high image quality, and glass replacement displays will not enter the market unless they are competitive in terms of image quality, power consumption, and cost. Consequently, we regard active matrix as being a prerequisite for plastic mobile displays, at least for the mass-market applications. The favoured display effect for these displays at the present time is transflective LCD, in order to get both high quality images in the backlit mode, as well as visibility in sunlit conditions. This eliminates the possibility of using opaque metal foils, or coloured polymer substrates for the active matrix back plane. It would only be possible to use such substrates for LCDs if good image quality can be achieved using a reflective effect combined with a good front light technology. Such displays have not yet been demonstrated, even using glass substrates. There is
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